You have to set the Sleep prefs to "Never." That way, you can launch iTunes, put on a tune, and close the lid of the Powerbook - and the Powerbook shouldn't go to Sleep and/or disturb the playing of music via iTunes.
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The Powerbook should still go to sleep if you set the preference to Never. The Powerbook is designed to sleep unless there is an external keyboard, mouse, and monitor attached to the Powerbook. I would not recommend closing the lid on the Powerbook due to the excessive amount of heat created when "using" the Powerbook. I've tried this setup before at home with the Powerbook on a small desk beside mine, I found it interesting, but when I touched the side of the Powerbook next to the power adapter, she was a little more toasty than usual.
The heat issue has not been relevant since the Wallstreet G3 laptops (which were the only Apple laptops of relatively recent vintage to vent heat through teh kbd.). Powerbooks are made to function with the clamsheel closed when connected to a second monitor.
However, you must have a power supply connected in order for this to work as well (Apple has some TIL with mumbo jumbo about larger power drain when connected to an external monitor).
This avoids any use of the machine as in iPod (after all, they'd prefer that you buy one of those [img]smile.gif[/img] ).
I had a long ongoing thread over at MacNN trying to "solve" this "issue" with no real success. The closest we were able to come was under OS 9 to have a game (Snood) that allowed over riding of sleep on in the foreground. Unfortunately this meant only one song would play b/c the game also over rode calls from all background apps.
I did find out that you could trick the machine into thinking it was connected to an external monitor (again this was tested in OS 9 only) by booting it up with a VGA monitor plugged in and then diconnecting the monitor, the only problem then was that as soon as you d/c'd the power, the machine would go to sleep.
So here's a (very very large) kludge you can try. Haul a VGA monitor out you your car and plug it in, connect you laptop to your DC outlet using a power converter. Boot the machine (in OS 9, or you can try X but I don't know if it will work) and get iTunes running. Unplug the monitor and it should run (with the cassette adapter or what have you hooked to the car stereo).
Not going on a car trip? SOL I'm afraid.
Personally, for long car trips I've just put a folded cloth b/w the screen and the kbd and run it that way (makes it easier to change tunes since the kbd is live and you can use the arrow keys to navigate without having the screen lit).
So it doesn't look like there is any answer to this question. I used the Canadian tire $40 adapter to power the pb, a cassette adapter, put it level and open beside me and rocked on for the trip. Cool. Coming home I changed the itunes EQ to low bass since the growl was real bad with the previous setting. I was alone so it worked, I can't see my wife being happy in the back so the pb can ride up front.